Add pattern matching support for records with `@optional` fields.

[cristianoc]

This is follows on from #5423.

Example:

type t = {x: int, @optional y: string}

let v = {x: 3, y: "ab"}

let q = switch v {
| {x: 3, y: Some("ab")} => true
| _ => false
}

There's asymmetry between expressions and patterns.

Proposal: adopt for patterns the same convention used for expressions.

The example above now becomes:

let q = switch v {
| {x: 3, y: "ab"} => true
| _ => false
}

And for catch-all:

• Q: what would | {y: _} => mean? A: it means Some(_).
• Q: what does | {y : @optional _ } mean? A: it means _.

[rpominov]

What will be an alternative to | {x: 3, y: None} => true in the proposed syntax?

[cristianoc]

What will be an alternative to | {x: 3, y: None} => true in the proposed syntax?

Same as with expressions.
Btw this is not a syntax. That needs to be added on top.

[cristianoc]

The expression, which was like this before this addition of patterns, looks like this:

{x: 3, y: @option None}

[cristianoc]

In master.